The present invention relates to a method and apparatus for purifying exhaust gases from a diesel engine, more particularly the present invention relates to a method and apparatus for purifying exhaust gases from a diesel engine which is capable of removing particulates and NOx contained in the exhaust gases.
Present regulatory conditions in the automotive market have led to an increasing demand to improve fuel economy and reduce emissions in present vehicles. These regulatory conditions must be balanced with the demands of a consumer for high performance and quick response for a vehicle.
A diesel engine has an efficiency of up to about 52% and is thus the best converter of fossil energy. NOx emission concentration is dependent upon local oxygen atom concentration and the local temperature. Said high efficiency is however only possible at an elevated combustion temperature at which high NOx levels are inevitable. Moreover, a suppression of NOx formation by internal means (air/fuel ratio) has the tendency to cause an increase in particulates, known as the NOx-particulates trade off. Furthermore, an excess of oxygen in the exhaust gas from a diesel engine prevents the use of stoichiometric 3-way-catalyst technology for reduction of NOx as is used in gasoline engine cars from the late 80-ties.
Reducing the oxides of nitrogen (NOx) and particulate matter (PM) in exhaust gases from a diesel engine has become a very important problem in view of the protection of environment and the saving of finite fossil energy supply. For the coming legislation (US10, EU V1 etc) it may be necessary to have a combination of a diesel oxygen catalyst (DOC), a diesel particulate filter (DPF) and a selective catalytic reduction (SCR) catalyst in the exhaust system.
When optimizing in-cylinder combustion for maximum fuel efficiency generally gives high NOx levels in the exhaust gases. This is a problem with the nowadays stringent emission legislation, since the after treatment system can only reduce a certain amount of the NOx. Hence there is a trade-off between fuel efficiency and being emission compliant.
Lower engine-out NOx also reduces passive DPF regeneration. This may lead to an increased frequency of O2-based DPF regenerations (where applicable), which tends to deteriorate the EATS-system (DOC+DPF+SCR) at an accelerated rate. Hence less passive regeneration may lead to the need of having larger EATS-system, to compensate for increased deterioration. There is also an additional fuel penalty during the O2-based regeneration.
As explained above, there is a problem associated with prior art methods and apparatuses for purifying exhaust gases from a diesel engine.
It is desirable to provide an exhaust gas after treatment system and method which at least reduce the above mentioned problems.
According to a first aspect of the invention an exhaust gas after treatment system comprising a diesel particulate filter unit (DPFU) provided downstream of a diesel engine. A selective catalytic reduction (SCR) arrangement provided in fluid communication with said DPFU, A first injector for feeding reducing agent into the exhaust gas provided downstream of said DPFU and upstream of said SCR. At least one NOx sensor provided downstream of said SCR for providing at least one NOx signal to a NOx coordinator. At least one temp sensor provided upstream and/or downstream of said SCR for providing at least one temp signal to said NOx coordinator. Said NOx coordinator is provided for switching said diesel engine into a high NOx mode or a low NOx mode depending on the values of the at least one NOx signal and/or the at least one temp signal.
An advantage of an aspect of the present invention is that it maximises passive soot generation.
Another advantage of an aspect of the present invention is that the number of active O2 based regenerations may be kept to a minimum.
Still another advantage of an aspect of the present invention is that it allows for a low fuel and reducing agent consumption while maintaining a NOx emission below the legislation levels.
Still another advantage of an aspect of the present invention is that it allows the engine to be optimized for lowest fuel consumption at the right tail out NOx level.
Yet another advantage with an aspect of the present invention is that is also optimizes the usage of the SCR catalyst NOx conversion rate.
Still another advantage of an aspect of the present invention is that it allows the engine to be optimized for passive regeneration.
Still another advantage with an aspect of the present invention is that the number of active O2 based regenerations may be kept to a minimum.
Still another advantage with an aspect of the present invention is that it compensates for environmental effects (different conversion rates) and/or compensates for aging effects on the SCR and the engine.
Still another advantage with an aspect of the present invention is that it allows detection of problems with the SCR system.
Another advantage with an aspect of the present invention is that it will be possible to use a smaller SCR-catalyst, giving both cost, space and weight benefits.
The engine may be switched into said low or high NOx mode by changing for instance one or more of the following parameters: amount of exhaust gas recirculation (EGR), boost pressure, timing of fuel injection, fuel injection pressure, number of fuel injections. Said switching from high to low NOx mode may be performed when NOx signal is above a predetermined threshold value. Said switching from low to high NOx mode may be performed when NOx signal is below a predetermined threshold value and said temperature signal is between a first temperature T1 and a second temperature 12. Said switching from low to high NOx mode may also be dependent on the content of the NOx-reducing agent in the tank.
In an example embodiment of the present invention said DPFU comprising a DPF coated with an oxidation catalyst material. An advantage with that is that space, weight and cost may be further reduced.
In still another example embodiment a heat generator is provided between said diesel engine and said DPFU. An advantage with said embodiment is that optimal working temperature for the EATS may be achieved on demand irrespective of the load and RPM of the engine.
In still another example embodiment of the present invention a NO2 reduction catalyst is provided between in fluid communication with the DPFU and said SCR. An advantage with this embodiment is that NO/NO2 ration may be optimized for the SCR irrespective of the age of the EATS.
According to another aspect of the invention said exhaust gas after treatment method comprising the actions of: oxidizing NO into NO2 and trapping combustion particles in a diesel particulate filter unit (DPFU) provided in fluid communication with a diesel engine, reducing NO2 into NO in a selective catalytic reduction (SCR) arrangement arranged in fluid communication with said DPF, injecting a reducing agent by a first injector into the exhaust gas arranged downstream said DPFU and upstream of said SCR, providing a NOx signal to a NOx coordinator from at least one NOx sensor provided downstream of said SCR, providing a temperature signal to said NOx coordinator from at least one temperature sensor provided upstream and/or downstream of said SCR, switching said diesel engine into a high NOx mode or a low NOx mode depending on the values of the at least one NOx signal and/or the at least one temperature signal.
According to another aspect of the invention a computer program storable on a computer readable medium, comprising a program code for use in a method comprising the actions of: oxidizing NO into NO2 and trapping combustion particles in a diesel particulate filter unit (DPFU) provided in fluid communication with a diesel engine, reducing NO2 into NO in a selective catalytic reduction (SCR) arrangement arranged in fluid communication with said DPF, injecting a reducing agent by a first injector into the exhaust gas arranged downstream said DPFU and upstream of said SCR, providing a NOx signal to a NOx coordinator from at least one NOx sensor provided downstream of said SCR, providing a temperature signal to said NOx coordinator from at least one temperature sensor provided upstream and/or downstream of said SCR, switching said diesel engine into a high NOx mode or a low NOx mode depending on the values of the at least one NOx signal and/or the at least one temperature signal.
This computer program can be adapted to be downloaded to a support unit or one of its components when run on a computer which is connected to the internet.
In the drawings, equal or similar elements are referred to by equal reference numerals. The drawings are merely schematic representations, not intended to portray specific parameters of the invention. Moreover, the drawings are intended to depict only typical embodiments of the invention and therefore should not be considered as limiting the scope of the invention.